Recently, many vehicles are equipped with passenger protection apparatuses for protecting a passenger at the time of a collision of the same. For example, such passenger protection apparatus include passenger protection apparatus as defined in Safe-by-Wire Automotive Safety Restraints Bus Specification Version 1.0. The passenger protection apparatus includes a plurality of sensors, communication buses, and an electronic controller. Each of the plurality of sensors includes a switch therein and is daisy-chain-connected to the controller through a communication bus. At the time of initial setting, the switches in the sensors are sequentially connected to set addresses for all of the sensors sequentially based on address data serially transmitted from the controller. When the initial setting is completed, various data are serially transmitted between the controller and the plurality of sensors. Since addresses can be set at the time of initial setting as thus described, there is no need for setting an address specific to each sensor in advance. This is advantageous in that sensors can be used on a common basis.
To set addresses at the time of initial setting and to serially transmit various data after the initial setting is completed, the controller includes a communication circuit for each communication bus. It is most preferable to connect all sensors to one communication circuit through one communication bus because a simple configuration can be provided. In practice, however, a plurality of sensors must be connected to the controller through separate communication buses in most cases because of restrictions on the time for data transmit between the controller and the sensors and restrictions on communication bus wiring on the vehicle. For this reason, the number of communication buses is increased. As a result, the number of communication buses is also increased. This has made it difficult to keep the cost of a passenger protection apparatus low. It has been also difficult to provide a passenger protection apparatus in a small size.